1. Field of the Disclosure
The present application relates to lasers suitable for generating radiation at deep UV (DUV) and vacuum UV (VUV) wavelengths, and to methods for generating laser light at DUV and VUV wavelengths. In particular it relates to systems and methods for reducing and controlling the spectral bandwidth of DUV and VUV lasers. The lasers are particularly suitable for use in inspection systems including those used to inspect photomasks, reticles, and semiconductor wafers.
2. Related Art
The integrated circuit industry requires inspection tools with increasingly higher sensitivity to detect ever smaller defects and particles whose sizes may be about 100 nm or smaller. Furthermore these inspection tools must operate at high speed in order to inspect a large fraction, or even 100%, of the area of photomask, reticle or wafer, in a short period of time, e.g. one hour or less.
Generally short wavelengths such as DUV and VUV wavelengths have higher sensitivity for detecting small defects compared with longer wavelengths. Inspection of a photomask or reticle is preferably done using the same wavelength as used the lithography used when printing from the photomask or reticle. Currently a wavelength of substantially 193.4 nm is used for the most critical lithography steps and a wavelength of substantially 248 nm for less critical lithography steps. Where a wavelength value is mentioned herein without qualification, it should be assumed that value refers to the vacuum wavelength of the light or radiation.
High-speed inspection requires high power lasers in order to illuminate the samples being inspected with high intensity in order to detect the small amount of light scattered from small particles or defects or allow detection of small changes in reflectivity due to defects in the pattern. The required laser power levels may range from approximately 100 mW for the inspection of photomasks and reticles up to more than 10 W for the detection of small particles and imperfections on a bare silicon wafer.
Typically inspection in the semiconductor industry requires lasers with very narrow bandwidth. Such inspection systems usually use an objective lens with a large field of view (typically from a few hundred microns to a few mm in dimensions) in order to allow imaging of a large area to achieve high inspection speeds. An objective lens with low distortions and a large field of view is expensive and complex. Requiring that objective lens to operate over a large bandwidth (such as more than a few tens of μm) significantly increases the cost and complexity. DUV lasers with bandwidths of approximately 20 μm or less are very desirable for inspection applications in the semiconductor industry.
DUV lasers are known in the art. U.S. Pat. No. 5,144,630 entitled “Multiwave Solid State Laser Using Frequency Conversion Techniques” that issued on Sep. 1, 1992 to Lin and U.S. Pat. No. 5,742,626, entitled “Ultraviolet Solid State Laser Method Of Using Same And Laser Surgery Apparatus”, issued on Apr. 21, 1998 to Mead et al. describe exemplary DUV lasers. Fourth and fifth harmonics are generated from a pulsed fundamental infra-red laser operating at a wavelength near 1064 nm, thereby resulting in wavelengths of approximately 266 nm and 213 nm. Lin and Mead also teach generating an infra-red wavelength longer than 1064 nm from the fundamental laser using an optical parametric oscillator (OPO).
The output bandwidth of a laser oscillator is determined by its intra-cavity dynamics. In prior-art pulsed lasers, to further reduce laser bandwidth, various bandwidth limiting devices, such as an etalon, a birefringent filter, or an optical grating, have been incorporated into a laser cavity. Because all of these approaches are invasive, they inevitably introduced detrimental effects to the lasers. These detrimental effects include extra power losses and greater complexity, which often led to lower laser efficiency, poor thermal stability, tighter misalignment sensitivity, and longer laser system warm-up time. Furthermore, because intra-cavity beam size is often small and predetermined by the laser cavity design, and intra-cavity laser power density is normally much higher than laser output power, these intra-cavity components are much more susceptible to damage.
In prior-art pulsed DUV lasers, the bandwidth of the DUV output depends directly on the bandwidth of the fundamental infra-red laser. That is, the broader the bandwidth of the fundamental laser, the broader the DUV broader the DUV output bandwidth. Reducing the bandwidth of a laser requires redesigning the laser oscillator cavity. Since the cavity may control many properties of the laser including bandwidth, repetition rate, as well as average and peak powers, redesigning the cavity to reduce the bandwidth while maintaining the other laser parameters may be a complex and time consuming task. Furthermore it may not be possible to achieve a specific DUV laser bandwidth specification using a readily available infra-red fundamental laser.
Reducing bandwidth by frequency doubling by combining two femtosecond pulses with opposite chirp is known in the art (see Raoult et al., Opt. Lett. 23, 1117-1119 (1998)). A femtosecond pulse was first chirped and stretched to about ins using a grating-pair stretcher and then, after amplification, split into two pulses. The two pulses were incompletely compressed into tens of picosecond pulses with opposite chirp by using two grating-pair dispersers. Sum frequency generation of these two pulses resulted in a much narrower bandwidth. However, this approach relies on grating-based stretchers and compressors, which are bulky and lack the mechanical stability needed for demanding commercial industrial applications. Furthermore femtosecond pulses are generally unsuited to use in semiconductor inspection applications as the wide bandwidth (multiple nm) greatly complicates the design of the system optics, and the high peak power can easily damage the article being inspected.
Therefore, a need arises for DUV laser overcoming some, or all, of the above disadvantages. In particular a need arises for a means of reducing or controlling the bandwidth of a DUV laser, including DUV lasers with pulse lengths of between a few picoseconds and a few hundred picoseconds.